1. The Field of the Invention
The present invention relates to methods for digital signal processing and to transmission/reception systems that utilize said methods.
2. The Relevant Technology
The invention is intended mainly, but not exclusively, for receiving and transmitting digital audio and video signals, in particular those involved in the broadcasting of second-generation digital terrestrial television signals.
In order to protect the signals from the distortions of the transmission channel, the second-generation systems for broadband satellite broadcasting (DVB-S2) utilizes the LDPC (Low Density Parity Check) encoding associated with the QPSK, 8 PSK, 16 APSK and 32 APSK modulations (FIG. 1), which are suitable for transmission over a non-linear channel such as the satellite one. A description of the DVB-S2 standard and LDPC codes can be found, for example, in A. Morello, V. Mignone, “DVB-S2: The Second Generation Standard for Satellite Broad-band Services”, Proceedings of the IEEE, Volume 94, Issue 1, January 2006, Pages 210-227.
For the purpose of better exploiting the potentiality of the codes, the DVB-S2 standard provides that an interleaver is interposed between the LDPC encoder and the 8 PSK, 16 APSK and 32 APSK constellation mapper in order to achieve an improved association between the bits of the encoded word and the bits carried by the constellation points.
In the interleaver defined in the DVB-S2 standard, the encoded packet outputted by the LDPC encoder (formed by a number of bits equal to 16,200 or 64,800, which number is generally referred to with the symbol “NFRAME”) is written by columns in a matrix having N columns, where N is the number of bits carried by the constellation (N is 3 for 8 PSK, 4 for 16 APSK, 5 for 32 APSK), and NFRAME/N rows (FIG. 2), and is read by rows; reading takes place from left to right for all code rates provided by the standard, with the exception of the 3/5 rate, where reading takes place from right to left. The association with the constellation points or coordinates takes place as shown in FIG. 1.
For receiving and transmitting numerical audio and video signals involved in the broadcasting of second-generation digital terrestrial television signals, it has recently been thought of using the same encoding scheme as that employed in the DVB-S2 standard, i.e. the same LDPC codes, however associated with QAM (Quadrature Amplitude Modulation) modulations, in particular with the QPSK, 16 QAM, 64 QAM and 256 QAM modulations (FIGS. 3A-3D).
The Applicant has realised that, with QAM modulations, the performance offered by the LDPC codes are good but not wholly satisfactory as to the signal-to-noise ratio (SNR) required for reaching the QEF (Quasi Error Free) condition; as known, such a condition corresponds to the case wherein less than one error is received per hour of received program.
The general object of the present invention is to solve the above-mentioned problem and, in particular, to improve the association between the bits outputted by the LDPC encoder and the constellation coordinates of QAM modulations; more particularly, the present invention deals with the LDPC encoding with a 3/5 code rate and with the 16 QAM or 64 QAM or 256 QAM modulation.
Said objects are achieved through the methods for processing digital signals and the transmission and reception systems having the features set out in the appended claims, which are intended as an integral part of the present description.